Various types of semiconductor devices are manufactured in much the same way. A starting substrate, usually a thin wafer of silicon or gallium arsenide, is polished and the wafer is then masked, etched, and doped through several process steps, the steps depending on the type of devices being manufactured. This process yields a number of die on each wafer produced. The die are separated with a die saw, and then packaged into individual components.
The component package provides a means of input/output between the semiconductor die and the electronic device into which the die is installed, thereby allowing the host to read and/or write information to and from the die. The package usually comprises a main body and conductive leads. The body, manufactured most commonly of plastic or ceramic, protects the fragile die encased inside the body from contact with objects or solvents that could physically damage or corrode the circuitry on the die surface. Bond pads on the die are coupled with the leads, and the leads exit the main body to the exterior of the package.
Many different package types are available on the market, each providing advantages for use in different circumstances. Dual In-Line Packages (DIPs) have two rows of leads which, when inserted into a printed circuit board (PCB), pass through holes in the PCB. Single In-line Packages (SIPs) have a single row of leads and also pass through holes in the board.
Surface mount packages are becoming widely used in industry, primarily because they are more easily handled by automated equipment than packages with through-hole leads, and they can be mounted to both sides of a PCB. The leads of surface mount packages do not pass through the PCB, but instead attach to its surface by means such as solder. "Gull Wing" and Small Outline J-Lead (SOJ, or J-leads) are two styles of surface mount packages.
Semiconductor packages, in addition to being processed from various materials and with different lead types can have different body styles, such as a "chip carrier" style. Ceramic bodies are often manufactured as a leadless chip carrier (LCC), which is a rectangular package having conductive pads along either edge. An LCC is typically soldered directly to the PCB, or leads are sometimes attached to the package before the die is mounted to the package, then the device is mounted on the PCB. A ceramic LCC is typically manufactured by a package supplier, and then delivered to a semiconductor manufacturer. The semiconductor manufacturer then attaches the semiconductor die to a surface of the package, attaches bond wires, and seals the package with a metal lid.
Another style of semiconductor body is a bottom brazed flat package, as shown in FIG. 1. This type of package has a notch 10 along the length of either edge of the package body which results from the bottom half of the package being narrower than the top half. The flat package has conductive pads 12 which extend from the upper vertical portion of the edge of the package to the horizontal portion of the notch 10. Leads 14 can then be brazed to either the conductive pads on the side 12A, or to the pads 12B on the top of the notch 10. A ceramic flat package is generally manufactured with the leads by a package supplier, then shipped to the semiconductor maker who places the package in a frame for handling to prevent damage to the leads. The semiconductor manufacturer installs a die in the package, attaches bond wires to electrically couple bond pads on the die to traces manufactured into the package, seals the die in the package with a metal lid, and then trims and forms the leads to produce a gull wing flat package device. FIG. 2 shows a flat package having gull wing leads 20 brazed to the portion of the conductive pads 12B on the top of the notch 10, and the lid 22 which is added after the die is bonded to the inside of the package.
Packages, as previously mentioned, are available with both plastic bodies and ceramic bodies. Often the same lead styles, for instance DIPs, are available with both plastic and ceramic bodies. The way the lead types are formed on the ceramic and plastic bodies, however, must often vary between the ceramic and plastic implementations. One reason for this is that the plastic bodies use a lead frame, while ceramic bodies often do not. In plastic encapsulation methods, the die is bonded to the lead frame, bond pads on the die are wire bonded to the leads on the lead frame, and then the die and a portion of the leads are encapsulated in plastic. The encapsulation step is generally performed by the semiconductor manufacturer. The leads of the device extend from the inside of the body to the outside, thereby allowing a means of I/O between the die and the host. In ceramic embodiments, the leads are usually attached by the package supplier by brazing them to the pads on the surface of the ceramic body before the die is inserted into the package, then the package is shipped to the semiconductor manufacturer. FIG. 3 shows a ceramic package having DIP leads 30 brazed to conductive pads 32 on the side of the package. The semiconductor manufacturer bonds the die 34 to a shelf 36 in the package body and bonds the pads 38 on the die 34 to bond pads 40 located on a second shelf 42. This couples the bond pads 38 on the die 34 with conductive traces (not shown) which extend through the ceramic body to the conductive pads 32 on the outside of the package, and to the leads 30.
J-leads are implemented with plastic bodies by forming the leads from a lead frame. The die is first bonded to the lead frame, bond wires are electrically interposed between the bond pads on the die and lead fingers on the lead frame, and the die and a portion of the leads are encapsulated in plastic. The leads are trimmed to the correct length and then formed into SOJ style leads.
The implementation of J-leads with ceramic bodies has taken several forms. A first method of forming J-leads on ceramic packages, as shown in FIG. 4, has been by side brazing the leads 44 on the conductive pads 32 along either side of the package body. While this type of J-lead provides a fairly low profile package, it has disadvantages. One disadvantage to this package is that the leads are attached to the package and formed by a package manufacturer before it is shipped to the semiconductor manufacturer. This requires that the package pass through assembly and test after the die is installed with the leads attached to the package, which makes it difficult to maintain lead coplanarity and can reduce yields from package defects.
One advantage of J-leads over other types of surface mount options such as LCC's is that J-leads are more resilient, as they allow more "spring" once they are attached to the PCB. This spring reduces problems of thermal mismatch between the PCB and the ceramic package, which can cause an open in the electrical connection between the die and the PCB. A J-lead which is brazed onto the vertical side of the package, however, has reduced resiliency over J-leads of plastic packages which exit the package and bend to form the J-lead. The curve of the lead is essential in maintaining the flexibility advantage of the J-lead, which is missing in the side brazed lead attachment means.
A second implementation of J-leads on a chip carrier involves attaching copper J-leads to the pads by thermocompression. The advantage of this package over one which is side brazed is that the leads are assembled after the device is assembled and tested, thereby reducing the number of rejects from damaged leads. To attach these leads after assembly of the module, however, requires a that the leads be relatively long, thereby providing a higher profile package than is desirable.
A third method of implementing J-leads, as shown in FIG. 5, uses "claw type" leads 50 which are snapped onto the package body, then soldered to the conductive pad (not shown). This process also produces a part with a high profile, and requires that an insulative material 52 be interposed between the clip 50 and the metal of the package lid 22 to prevent shorting of the leads 50 by the metal lid 22.
No reliable means of fabricating a J-lead using the flat package is yet available. As a result, the flat package is available mainly with gull wing style leads, which provides the surface mount capability and spring of the J-lead, but does not provide the small outline of the J-lead and therefore requires more space on the PCB.
The mechanical flexibility of the J-lead which provides protection against problems of thermal expansion is a result of its shape. In the plastic embodiment, the J-lead exits the package in a horizontal direction, has a rounded shoulder which bends the lead downward at approximately 90.degree. to a vertical direction, and has a vertical section different lengths for different J-lead embodiments. The lead then curves inward toward the center of the package and usually bends back to the vertical direction toward the bottom of the package, although the lead may not reach the vertical direction in some embodiments. Several embodiments of J-leads are possible, and FIG. 6 shows a sampling of different J-lead styles.
J-leads which are side brazed, attached by thermocompression, or a claw type, have the disadvantages listed above, and further do not have the curved shoulder which gives a J-lead of a plastic package its resiliency. A J-lead on a flat package having a curved shoulder would have advantages over the J-lead mounted to the side of the chip carrier package by providing a better thermal expansion buffer between the PCB and the ceramic body. It would also have advantages over a gull wing flat package in that the outline would be smaller, thereby allowing for a smaller device footprint and more devices to be installed on a PCB.